According to a previously known technique, in a solenoid device, a plunger is moved in an axial direction along an inner wall of a stator core shaped into a cylindrical tubular form through energization of a solenoid. According to this technique, the cylindrical tubular form of the stator core is adjusted to adjust a change in an attractive force for attracting the plunger in conformity with a stroke position of the plunger.
For example, in the solenoid drive device disclosed in JP6094309B2 (corresponding to US2015/0179322A1), at the core main attracting portion of the solenoid, a conical surface, which passes two points, i.e., a start point of a taper from a magnetic flux limiting thin wall portion and an end point of the taper along a cylindrical tubular portion (the end point of the taper corresponding to an ON-position of the plunger), is defined as an imaginary conical surface that serves as a reference. Here, the core main attracting portion is configured such that a radially inner side of the imaginary conical surface is in a form of a recess, and a radially outer side of the imaginary conical surface is in a form of a projection. In this case, the core main attracting portion is configured into a taper form that includes the recess, which is located on the radially inner side of the imaginary conical surface, and the projection, which is located on the radially outer side of the imaginary conical surface.
Furthermore, in an electromagnetic valve device disclosed in JP3757817B2 (corresponding to US2002/0175569A1), an increase rate of a diameter or an angle of a taper segment at a core main attracting portion of a solenoid decreases in an ON-side moving direction. Alternatively, the taper segment is tapered in a plurality of steps or is in a convex curved surface such that an angle of the taper of the taper segment is reduced in the ON-side moving direction.
A solenoid device, which is applied to the fluid control valve, will be described as an example. In the configuration of the core main attracting portion according to the previously proposed technique, an attractive force characteristic is set such that a high attractive force is obtained at a center section, which is spaced from the OFF-end of the stroke toward the ON-side, i.e., the high attractive force is obtained in a range for controlling the fluid with the valve, and a curve of the attractive force characteristic is in a form of a mountain along the entire stroke range. Furthermore, according to the technique of JP3757817B2 (corresponding to US2002/0175569A1), a relatively flat attractive force characteristic is obtained by increasing the attractive force at the OFF-end side and the ON-end side. It is considered to be beneficial in terms of fluid control with respect to that the amount of change in the attractive force relative to the stroke movement of the plunger becomes small.
However, in the previously proposed technique, the attractive force characteristic becomes such that the attractive force is progressively increased from the OFF-end of the stroke to the center section. With the progressively increasing attractive force characteristic described above, the attractive force at the OFF-end is small at the time of applying a step current of ON to the solenoid, e.g., at the time of applying a response current that shows a first order lag characteristic due to an inductance or a feedback control operation of a drive circuit. Therefore, the time of starting the movement of the plunger from the OFF-end by overcoming the urging load of the spring is delayed. Since the time difference between the time of starting the movement of the plunger from the OFF-end and the time of starting the control operation of the fluid becomes small, the moving speed of the movable member having the weight (e.g., the plunger and/or the valve) at the time of starting the control operation of the fluid, i.e., the time of starting a change in the output characteristic becomes very high. Therefore, due to the high inertial force, which is applied to the movable member, the pressure or the flow rate, which is supposed to be stably controlled, does not immediately converge to the predetermined target value thereof. Thus, there is created an inconvenience, such as generation of overshooting and/or chattering that respectively serve as a transient characteristic, or an inconvenience of forming a start point of continuous pulsations.